In a dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter simply also referred to as an ink sheet) containing a colorant (hereinafter simply also referred to as a dye) is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter simply also referred to as an image-receiving sheet), and then the heat-sensitive transfer sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the heat-sensitive transfer sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, or four colors which consist of the three colors and black are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
As a method to obtain a thermal transfer image-receiving sheet, for example there is a known method in which a heat insulation layer and a receptor layer are sequentially formed on a substrate sheet by a gravure coat or the like. However, in this method, since each layer is sequentially formed, there is a problem that a number of steps is increased. Thus, in order to obtain the thermal transfer image-receiving sheet with a small number of the steps, a method or the like, in which layers are simultaneously formed, attracts an attention.
For example, as described below, there are known heat transfer image-receiving sheets, which are produced by simultaneously multilayer-coating the coating liquids (aqueous coating) for each layer on a support.
JP-A-2008-105366 (“JP-A” means unexamined published Japanese patent application) discloses an aqueous heat transfer image-receiving sheet, provided on a substrate sheet, with a porous layer (heat insulation layer) that imparts a heat insulation property, a primer layer (intermediate layer) that enhances the adhesiveness between the porous layer and a receptor layer and improves the surface property of the receptor layer, and the receptor layer, by a slide coating method.
JP-A-2008-162155 discloses that a primer layer is similarly provided between a receptor layer and a porous layer, but is characterized by providing the receptor layer by a solvent system.
JP-A-2009-83273 discloses a heat transfer image-receiving sheet provided with a substrate sheet, a porous layer containing a cooling gelling agent, and a receptor layer in this order, in which a cushion layer containing an SBR resin and a gelatin is provided in at least one of the site between the substrate sheet and the porous layer and the site between the porous layer and the receptor layer.
In the case where the heat insulation layer of the heat-sensitive transfer image-receiving sheet is constructed by the aqueous coating, it is necessary to fill the heat insulation layer with foamed hollow particles as far as possible, in order to secure a desired heat insulation property. However, if the content of the hollow particles is increased, the heat insulation layer is weakened and a cushioning property is decreased, and in addition, the disturbance at the interface between the heat insulation layer and the layer disposed thereon, is increased.
There is known a heat-sensitive transfer image-receiving sheet having the intermediate layer disposed between the receptor layer and the heat insulation layer such as described above, so as to mitigate the disturbance at the interface. However, in that case, the insertion of one intermediate layer inevitably causes an enlargement of the distance from the outermost surface of the receptor layer to the heat insulation layer. For that reason, the heat insulation property upon printing is weakened, and thereby the amount of dye transfer is decreased. Thus, the heat-sensitive transfer image-receiving sheet is unsuitable for a high speed image printing.
Furthermore, it has been preferable traditionally or in low speed printers such as sublimation type printers for consumer use, that the glass transition temperature (Tg) of a polymer in the intermediate layer is rather lower. However, in sublimation type printers for business in recent years, the temperature of the thermal head is set high, concomitantly with an acceleration of a printing speed. Thus, if the glass transition temperature (Tg) of the polymer in the intermediate layer is low, there occurs a problem that the intermediate layer is softened by the heat transferred from the head and loses its elasticity, and as a result, the pressure exerted on the hollow particles in the heat insulation layer is increased, causing destruction of the hollow particles and a consequent decrease in a sensitivity.
In the sublimation type printers, the heat-sensitive transfer image-receiving sheet is pressed from the rear side against the thermal head with a rubber roller called platen roller, in order to increase the adhesiveness among the transfer sheet, the image-receiving sheet and the thermal head. There is also a problem that, along with an increase in the number of printed sheets, the circularity of the platen roller is decreased, and the unevenness of the pressure that is exerted on the thermal head is increased, and color unevenness strongly appears in a longitudinal direction.